Governors: senators (or knights) who ruled the provinces of the Roman empire.
The first Roman province, Sicily, was conquered after the First Punic War (241 BCE), and the Senate decided that it had to be ruled by a praetor. This meant that civil (not military) law was applied -at least under normal circumstances- and that the new territories were governed by magistrates who served a limited time. The Romans never did change these principles, although other types of governorship became more important: the propraetor and proconsul were, as their names suggest, former praetors and consuls who stayed in a territory they had recently or not yet fully conquered. The revolutionary politician Gaius Sempronius Gracchus legislated that these promagistrates were to be appointed by the Senate (123 or 122).
The governor of any Roman province always had four tasks.
To start with, he was responsible for the taxes. As the Senates financial agent, he had to supervise the local authorities and the private tax collectors, the notorious publicans. To facilitate things, a governor could mint coins and negotiate with wealthy institutions (e.g., temples) that could advance the money. His second task was that of accountant: he inspected the books and supervised large scale building projects. Next to these financial tasks, the governor was the provinces supreme judge. Appeal was not impossible, but the voyage to Rome was expensive. He was supposed to travel through the main districts of his province to administer justice in the assize towns. Finally, he commanded an army. In the more important provinces, this could consist of legions; but elsewhere, there were only auxiliaries.

Under the late republic, the number of provinces rapidly increased, and therefore, Pompey the Great proposed a new law, the Lex Pompeia de provinciis , in which former praetors and consuls were obliged to become governor five years after their term in office (53). At more or less the same time, he had himself elected as governor of several provinces, which were not governed by himself, but by his representatives, the legati .
The emperor Augustus copied this idea when he changed the empire, until then ruled as a republic, into a monarchy. He was made governor of almost all provinces with legions, and used legati to rule them. At the same time, the rest of the empire was governed by proconsuls. So, there were two types of governors:
Proconsuls. In fact, these men were not former consuls, but former praetors. They governed the senatorial provinces and typically served twelve months. Only the rich provinces -Asia and Africa- were entitled to a proconsul who was indeed an ex-consul. Legati Augusti pro praetore. These men served in the emperors provinces with the armies (the imperial provinces ). Usually, their term in office lasted thirty-six months, although the emperor Tiberius preferred longer terms.
There was a third group of governors. In several unimportant provinces, prefects were appointed. Usually, these military men governed parts of larger provinces. The best known example is Pontius Pilate, who governed Judaea, an annex to Syria. Prefects were not senators but knights. Egypt was also governed by a prefect, not because it was unimportant, but because it was the emperors own possession. When Septimius Severus conquered Mesopotamia, he used the same construction.
After the mid-first century, the prefects were gradually replaced by procurators (except for Egypt). The only difference is that prefects were soldiers and procurators were fiscal officials. It tells something about the success of the Pax Romana .